Configuring OSPF route summarization
Route summarization is when an ABR or ASBR summarizes routes with the same prefix into a single route and distributes it to other areas.
Route summarization reduces the traffic of routing information exchanged between areas and the sizes of routing tables on routers, improving route calculation speed on routers.
For example, there are three internal routes in an area: 19.1.1.0/24, 19.1.2.0/24, and 19.1.3.0/24. By configuring route summarization on the ABR, the three routes are summarized into the route 19.1.0.0/16 that is advertised to other areas.
Configuring route summarization on an ABR
If contiguous network segments are available in the area, you can summarize them into a single network segment. An ABR generates Type-3 LSAs on a per network segment basis for an attached non-backbone area.
The ABR in the area distributes only the summary LSA to reduce the scale of LSDBs on routers in other areas and the influence of topology changes.
To configure route summarization on an ABR:
Step | Command | Remarks |
|---|---|---|
1. Enter system view. | system-view | N/A |
2. Enter OSPF view. | ospf [ process-id | router-id router-id | vpn-instance vpn-instance-name ] * | N/A |
3. Enter OSPF area view. | area area-id | N/A |
4. Configure ABR route summarization. | abr-summary ip-address { mask | mask-length } [ advertise | not-advertise ] [ cost cost ] | Not configured by default. The command is available on an ABR only. |
Configuring route summarization when redistributing routes into OSPF on an ASBR
Without route summarization, an ASBR advertises each redistributed route in a separate ASE LSA. After a summary route is configured, the ASBR advertises only the summary route in an ASE LSA instead of more specific routes, which reduces the number of LSAs in the LSDB.
The ASBR summarizes redistributed Type-5 LSAs that fall into the specified address range. If the ASBR is in an NSSA area, it also summarizes Type-7 LSAs that fall into the specified address range. If the ASBR is also the ABR, it summarizes Type-5 LSAs translated from Type-7 LSAs.
To configure route summarization when redistributing routes into OSPF on an ASBR:
Step | Command | Remarks |
|---|---|---|
1. Enter system view. | system-view | N/A |
2. Enter OSPF view. | ospf [ process-id | router-id router-id | vpn-instance vpn-instance-name ]* | N/A |
3. Configure ASBR route summarization. | asbr-summary ip-address { mask | mask-length } [ tag tag | not-advertise | cost cost ] * | The command is available on an ASBR only. Not configured by default. |